In recent years, heat-sensitive recording methods have grown in popularity in the facsimile and printer fields. As heat-sensitive recording materials for use in such recording methods, leuco color type heat-sensitive recording materials of excellent color density of and rate of color formation have been mainly used. However, conventional leuco color type heat-sensitive recording materials have the disadvantage in they readily form color upon handling after recording, heating or contact with solvents, thereby yielding a smudged recorded image. Further, they have the disadvantage that the color formed disappears due to the action of plasticizers in adhesive tapes (e.g., Cellotape).
In order to prevent color formation due to careless handling, Japanese Patent Publication No. 14531/75 discloses adding granulated wax and Japanese Utility Model Application Laid-Open No. 125354/81 discloses providing a covering layer to prevent plasticizer permeation. These methods, however, are not satisfactory, and cannot be used particularly for purposes in which alteration after recording must be prevented.
In order to prevent color formation in undesired areas after thermal recording, Japanese Patent Application (OPI) No. 91438/84 (the term "OPI" as used herein indicates a "published unexamined Japanese patent application") discloses a light-sensitive, heat-sensitive recording material in which microcapsules containing a photopolymerizable vinyl compound, a photo-polymerization initiator and one component causing a color formation reaction are used with another component capable of forming a color formation reaction with the above component which is exterior the microcapsules, all being provided on the same side of the support. Upon heating this recording material, the color forming component contained in the inside (core) of the microcapsules permeates through the microcapsule walls or the other component causing the color formation reaction exterior the microcapsules permeates through the microcapsule walls and enters the microcapsules. As a result, in both cases, color formation occurs. Accordingly, heating permits color formation in heated areas. By then applying overall light-exposure to polymerize the vinyl compound contained in the core of the microcapsules, permeation of the color forming component is prevented and thus color formation in uncolored areas can be prevented (this is sometimes called "fixation").
Japanese Patent Application (OPI) Nos. 123086/82 and 125092/82 (corresponding to U.S. Pat. No. 4,411,979), for example, disclose another method. In accordance with this method, a light-sensitive, heat-sensitive recording material containing a diazo compound, a coupling component and an alkali generating agent or a color forming aid is used; this material is overall irradiated with light after thermal recording to thereby decompose unreacted diazo compound, whereby color formation can be stopped. This recording material, however, has the disadvantage in that pre-coupling gradually proceeds during storage of the material, causing undesirable color formation (fog). In order to eliminate this problem, Japanese Patent Application (OPI) No. 190886/84 discloses a material in which at least one of the diazo compound, the coupling component and the color forming aid is incorporated in the inside (core) of the microcapsule.
With the above light-sensitive, heat-sensitive recording materials utilizing microcapsules, recording apparatus, including a light fixing unit, can be simplified, and the storage stability before recording (life storage stability) is excellent.
In the case of a system not including the capability for the above-described fixation, though based on microcapsules, i.e., a heat-sensitive recording material which contains a basic dye precursor in microcapsules and a developer causing a color formation reaction with the precursor present exterior the microcapsules, image storage stability (other than heat resistance) is excellent because the color forming dye after thermal recording is present in the interior of the microcapsules.
In the case of a heat-sensitive recording material based on microcapsules which has excellent recorded images storage stability, the color forming component isolated by the microcapsule walls permeates on heating, thereby reacting. Thus a reduction in heat color-forming properties is easily caused.